High vacuum



Patented Feb. 14, 1939 UNITED STATES 2,147,488 PATENT OFFICE amass men vacuum Kenneth C. D. Hickman and James G. Baxter,

Rochester, N. Y., assignors, by mesne assignments, to Distillation Products, Inc., Rochester, N. Y., a corporation oi Delaware No Drawing. Application October 23, 1936, Serial No. 107,256

3 Claims. (01. 230-101 This invention pertains to improved method and apparatus for the production of high vacua and, particularly, to the provision of improved organic pump'fluids for use in high vacuum pumps 5 of the general nature of diflusion or condensation pumps.

In the U. 3. Hickman Patents 1,857,506 and 1,857,508, thereare disclosed improved pumping have decidedly advantageous characteristics are 20 the phthalic acid esters of alcohols having from 6-9- carbon atoms, inclusive, and especially the esters of aliphatic alcohols having 7, 8 or 9 carbon atoms. Examples of such esters are:

. Din-hexyl phthalate Di n-heptyl phthalate Di n-octyl phthalate Di (ll-ethyl hexyl) phthalate Di n-nonyl phthalate Di (p-phenyl ethyl) phthalate Combinations in which two diflerent alcohols are reacted with the two carboxyl groups of the phthalic acid molecule may, of course, be used.

' Also isomeric forms of these alcohols give compounds having the same or similar desirable prop- 35 erties.

These phthalic acid esters can be prepared by' using any one of a variety of diiferent processes for ester production. One well known method 40 which has been found to give excellent yields is -to mix one molecular portion of phthalic anhydride with an equal weight of toluene and 2-3 molecular proportions of the app opriate alcohol.

This mixture is mechanically stirred in a round 45 bottom flask at a temperature below the boiling point of toluene until a clear solution results. A

quantity of para toluene sulfonic acid amounting to 1% of phthali'c anhydride is then added as a catalyst. The mixture is then heated to distilr,

5o lation temperature, water formed during the reaction being distilled off with part of'the toluene, the mixed vapors being condensed, stratifled, and the top layer of'toluene being returned to the reaction mixture Practically-complete esteriflcation is obtained in a out 7 hours using this method. The phthal acid ester is removed and washed with cold 10'7 aqueous sodium chloride to which was added a eachextraction a small amount of 10% sodium hydroxide containingan 69 indicator, such as phenolphthalein. Extraction is continued until the indicator color is no longer discharged indic ng that the excess phthalic anhydride has n neutralized. The esteriflcation mixture is then washed with water to remove the last traces of soap and dried over sodium sulfate. Toluene is separated from the ester thus formed by distillation at water pump vacuum and finally the esteris distilled at a pressure of about .1 of a mm. The esters prepared in this manner are pale yellow in color and quite mobile and are suitable for use as condensation pump fluids. Other methods of ester production well known in the chemical art can, of course, be used if desired.

These esters have surprising characteristics as condensation pump fluids. All-of them have a lower vapor pressure than those heretofore employed as pump fluids and for that reason enable the production of much higher vacua. For this reason cooled traps need not be used in most cases. In spite of their low vapor pressure they are nevertheless very resistant .to thermal decomposition such as often takes place during normalpumping action or as a result of accidental hot exposure at atmospheric pressure. such exposure is also very rapid and unless the exposure is greatly prolonged little damage takes place. They have a high. pumping rate and will produce rapidly a higher vacuum at 25 C. than any other organic or inorganic pump liquid heretofore known.

The improved fluids are used in condensation pumps of any design, but are preferably used in condensation pumps especially designed for organic fillings such as those disclosed in the Hickman patents above referred to and in the copending Hickman application #2'7,652 filed June The following tables indicate the improved pumpingqualities of these esters as regards degree of vacua obtainable and rate of conditioning.

Table I indicates. the minimum pressures obtainable with the improved pump fluids as compared with butyl phthalate. Thedeterminations were made with an uncooled trap held at 25 C. It is seen that our new pump fluids enable the production or very high vacua which heretofore required elaborate pumping equipment and cumbersome and expensive cooled traps. When Recovery from r cooled traps are used with our pump fluids a vacuum is rapidly established which is below the limits of measurement with known pressure meas uring devices.

. Table II Rate of ocn- Rate oi conditioning oi ditloning alter fresh flllings hot exposure n-Heryl phthalate ao minutes minutes n-Heptyl phthalats. 2 hours minutes n-Octyl phthslateu 14 hours 3 hours Hydrocarbon oil--. .5 72 hours 16 hours The rateof conditioning of a pump fluid is of importance since this property determines the amount of time required for the'pump to produce the maximum vacuum. During use in commercialoperatlon the pump will Often be accidentally exposed to atmospheric pressure causing partial or complete destruction of the pump fluid depending upon its resistance to such treatment. A pump fluid which will withstand hot exposure. and quickly reestablish a high vacuum after such treatment is a necessity in most pumping operations. Table II shows the rate of conditioning of fresh pump fluids and of pump fluids subjected to hot exposure for the indentical periods of time. The hydrocarbon oil is a well known condensation pump fluid produced by molecular distillation of a petroleum oil. It is seen that our improved fluids condition much more rapidly than the hydrocarbon pump fluids heretofore used.

Phthalic acid occurs in a plurality oi isomeric forms. The ordinary (ortho) form is preferred for use in preparing the improved pump fluids, but the other isomeric forms such as terephthalic acid are suitable and are to be understood as being within the scope of our invention.

What we claim is:

1. A means for evacuating closed vessels which comprises a diflusion or condensation pump containing a di octyl phthalate as an evacuating medium. '2 1 2. A means for evacuating closed vessels which comprises a diffusion or condensation pump con- I taining di-n-octyl phthalate as an evacuatingmedium.

3. A means for evacuating closed vessels which comprises a diflusion or condensation pumpcontaining di-2-ethyl hexyl phthalate as an evacuating medium.

KENNETH C. D. HICKMAN. JAMES G. BAX'I'ER. 

